1. Field of the Invention
The present invention relates to a broad band light absorbing photocatalyst which has a high absorptivity not only for visible light but also, in particular, for ultraviolet light and exhibits photocatalytic activity in response to a broad band light; a process for producing thereof efficiently; a broad band light absorbing photocatalyst composition which comprises the broad band light absorbing photocatalyst; and a molded article which comprises the broad band light absorbing photocatalyst.
2. Description of the Related Art
In recent years, photocatalytic activity, which exhibits oxidative decomposition effect, antibacterial effect, antifouling effect, etc., of certain semiconductor materials, e.g. titanium oxide (TiO2), has been paid attention to. In general, when the semiconductor materials having such photocatalytic activity absorbs light having an energy corresponding to the band gap between the valence band and the conductor band, the electron present in the valence band transits to the conductor band. The electron which transited to the conductor band have characteristics of moving to substances adsorbed on the surface of the semiconductor material having the photocatalytic activity and, when substances are adsorbed on the surface of the semiconductor material, the substances are reduced by the electron. Due to the transition, a hole is generated in the valence band. The hole generated in the valence band have characteristics of stripping the electron from the substance adsorbed on the surface of the semiconductor material having the photocatalytic activity and, when substances are adsorbed on the surface of the semiconductor material, the substances are oxidized as a result of the strip of the electron thereof by the hole.
To explain the above-mentioned phenomenon more specifically, taking titanium oxide having particularly excellent photocatalytic activity as an example, when titanium oxide absorbs light with energy equivalent to band gap between the valence band and conduction band, an electron in the valence band transits to the conduction band and the transited electron reduces oxygen in the air, thereby allowing generation of superoxide anion (•O2—). In contrast, as a result of the transition of the electron, a hole is generated in the valence band and the generated hole allows the generation of hydroxy radical (•OH) by oxygenating water adsorbed on the surface of the titanium oxide. The resulting hydroxy radical is strongly oxidative, and thus when organic substances or the like are adsorbed on the surface of the titanium oxide, the organic substances or the like are decomposed by the action of the hydroxy radical and finally decomposed to water and carbon dioxide. In this way, when the semiconductor materials having photocatalytic activity, such as titanium oxide, are exposed to light with energy equivalent to band gap between the valence band and conduction band, the semiconductor materials absorbs the light and organic substances or the like are decomposed. Thus, oxidative decomposition effect, antibacterial effect, antifouling effect, etc. are achieved.
Therefore, recently, the semiconductor materials having photocatalytic activity, particularly, titanium oxide, have been widely utilized as anti-fungus agent, bactericide, antifouling agent, deodorant, agent for environmental cleanup, or the like. For example, Japanese Patent Application Laid-Open (JP-A) No. 11-195345 have disclosed a technique to provide a push button of electronics device with antibacterial properties by applying photocatalytic titanium oxide to the push button. Further, JP-A 2003-305371 have disclosed a photocatalytic thin film containing particles having photocatalysis composed of metal elements which have an electronegativity of less than 1.6, ionic radius of less than 0.2 nm and atomic valence of 2 or less and an article provided with the photocatalytic thin film on the surface thereof.
These proposals, however, have the following problems. Specifically, light energy required to excite titanium oxide exhibiting excellent photocatalytic activity is 3.2 eV, which corresponds to about 380 nm in terms of the wavelength of light. This means that when the titanium oxide is exposed to near ultraviolet light, it is excited, but when exposed to visible light, it is not excited. Ultraviolet light constitutes a small fraction, i.e., only 4 to 5% of sunlight, causing one problem that when sunlight is used as an exposure light, the titanium oxide does not exhibit satisfactory photocatalytic activity. Further, another problem is that when exposed to fluorescent light, in which ultraviolet light is hardly present, in a room, the titanium oxide hardly exhibits photocatalytic activity.
It has been strongly demanded to develop titanium oxide which solves such problem that articles for use under sunlight or fluorescent light in a room cannot be provided with adequate photocatalytic activity and, at the same time, exhibits satisfactory photocatalytic activity when exposed to visible light which accounts for 45% of sunlight and accounts for most of the fluorescent light. Thus, investigations related to the response of the titanium oxide to visible light have been widely made. Therefore, investigations about the response of the titanium oxide to visible light have been widely conducted.
As an example of such investigations, several techniques in order to provide the titanium oxide with visible light response have been provided, for example, a technique in which oxygen deficiency is formed and a technique in which the titanium oxide is doped with nitrogen. In these cases, however, satisfactory outcome for practical purposes has not been achieved and the current status of research is still on the research stage.
On the other hand, the titanium oxide has low absorption ability towards materials. Thus it is required to improve absorption ability of the titanium oxide towards objects to be decomposed in order to make the titanium oxide to exhibit oxidative decomposition effect, antibacterial effect, antifouling effect, etc. based on its photocatalytic activity.
Therefore, a technique utilizing the properties of an apatite, as a material having excellence in such absorption ability towards objects to be decomposed, has been researched and developed. The apatite such as calcium hydroxyapatite Ca10(PO4)6(OH)2 which is a main component of the hard tissue of a living body such as teeth and bones, is likely to undergo the ion exchange with a variety of cations and anions, has high biocompatibility and adsorption property, and has specific adsorption ability towards organic substances such as proteins.
As an example of such researches and developments, JP-A Nos. 2003-80078 and 2003-321313 have disclosed products in which semiconductor materials such as titanium oxide is utilized in combination with calcium phosphate compounds such as calcium hydroxyapatite and the products can exploit properties of both efficiently. Further, JP-A Nos. 2000-327315, 2001-302220, 2003-175338, and 2003-334883 have disclosed titanium-calcium hydroxyapatite Ca9Ti(PO4)6(OH)2 having photocatalytic function where calcium ions in the apatite is partially ion exchanged, so-called photocatalytic titanium hydroxyapatite (Ti-HAP).
In the case of the photocatalytic titanium hydroxyapatite (Ti-HAP), however, there is also a similar problem as mentioned above that when fluorescent light in a room in which ultraviolet light is hardly present, the titanium oxide hardly exhibits photocatalytic activity.
Accordingly, a broad band light absorbing photocatalyst which has a high absorptivity not only for visible light but also, in particular, for ultraviolet light, exhibits photocatalytic activity in response to a broad band light over a long period, has a high adsorptivity for objects to be decomposed, can exhibit oxidative decomposition effect, antibacterial effect, antifouling effect, etc. and is high-performance; a process for producing thereof efficiently; and a variety of techniques to which the broad band light absorbing photocatalyst is applied, have not been provided yet and thus development thereof has been demanded.
An object of the present invention is to solve the above-mentioned conventional problems and to provide a broad band light absorbing photocatalyst which has a high absorptivity not only for visible light but also, in particular, for ultraviolet light, exhibits photocatalytic activity in response to a broad band light over a long period, has a high adsorptivity for objects to be decomposed, can exhibit oxidative decomposition effect, antibacterial effect, antifouling effect, etc. and is high-performance; a process for producing thereof efficiently; a broad band light absorbing photocatalyst composition which comprises the broad band light absorbing photocatalyst; and a molded article which comprises the broad band light absorbing photocatalyst.